The present invention is directed, in general, to wireless networks and, more specifically, to a system for performing an idle handoff of a mobile unit between base stations during a system access in a CDMA wireless network.
In 1996, more than 75 million people worldwide used cellular telephones. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered not only by dedicated cellular service providers, but also by the regional Bell companies, such as U.S. West, Bell Atlantic and Southwestern Bell, and the national long distance companies, such as ATandT and Sprint. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population.
Wireless subscribers use a wide variety of wireless devices, including cellular phones, personal communication services (PCS) devices, and wireless modem-equipped personal computer (PCs), among others. The large number of subscribers and the many applications for wireless communications have created a heavy subscriber demand for RF bandwidth. To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
Although TDMA-based wireless networks were developed on a large scale first, CDMA-based wireless networks are now also widely used. As is well known, CDMA systems divide the RF spectrum into a number of wideband digital radio signals. Each digital radio signal carries several different coded xe2x80x9cchannelsxe2x80x9d. Each coded channel is distinguished by a unique pseudo-random noise (PN) code used by the mobile station and/or the base station. In a CDMA receiver, the coded channels are decoded by a signal correlator that matches PN sequences. Some coded channels are used as data traffic channels to transport subscriber voice and/or data signals, while other coded channels are used as control channels, including pilot, synchronization, paging and access channels. Some CDMA systems may use an analog control channel to allow a subscriber""s wireless device (or mobile station) to access the CDMA system.
When a mobile station accesses a base station in a CDMA wireless network, the CDMA control channel (which has one unique PN code) assigns the mobile station to a data traffic channel (which has a different PN code) on which the mobile station exchanges data traffic with another party, a data terminal, a fax machine, or the like. Typically, the coded control channel and the data traffic channel(s)s used by the mobile station and the base station are on the same RF carrier frequency. Advantageously, in many CDMA networks, the control channels and the data traffic channels are on the same RF carrier frequency in all cells (i.e., base station coverage areas) in the networks.
Because adjoining base stations in the CDMA network are operating on the same RF carrier frequency, a mobile station can simultaneously access two (or more) base stations by using the same PN codes to communicate in data traffic channels and control channels with each base station. This enables a process known as xe2x80x9csoft handoffxe2x80x9d to occur when a mobile station communicating with a first base station enters a second base station""s cell site. A soft handoff may occur while a mobile station is actively communicating with another device (e.g., a voice phone call). A soft handoff also may occur when the mobile station is turned xe2x80x9cONxe2x80x9d and has accessed the CDMA network, but is not actively communicating with another device. This is known as an xe2x80x9cidlexe2x80x9d handoff.
In a CDMA wireless network, call setup failures frequently occur due to very quick propagation environment changes, such as when a mobile station moves behind a hill or a large building. This can cause problems when the mobile station is in the middle of a system access operation. For example, when a mobile station sends an Origination message to a first base station or receives a Page message from that first base station, the mobile station enters a xe2x80x9csystem accessxe2x80x9d state during which the mobile station cannot undergo a handoff to a second base station. If the RF propagation environment changes quickly enough, the mobile station may not receive a channel assignment message from the first base station and the data traffic channel will not be set up. In conventional CDMA networks, the mobile station cannot be put into a soft handoff at the time of initial channel assignment. A soft handoff setup is initiated only after a mobile station is assigned to a traffic channel and is sending a Pilot Strength Measurement message to the network. As a result, the attempted set up process with the first base station and all related messages are lost, and the mobile station must start over with another base station.
There is therefore a need in the art for improved CDMA wireless networks that allow a mobile station to perform an idle handoff while in a system access state in order to reduce the number of lost messages caused by changes in the RF propagation environment. In particular, there is a need in the art for improved CDMA systems that more rapidly initiate a soft handoff procedure for a mobile station to thereby reduce the call setup failure rate of the CDMA network. More particularly, there is a need for CDMA systems that can initiate a handoff procedure without waiting for a Pilot Strength Measurement message from the mobile station.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a CDMA wireless network, a base station capable of wirelessly communicating with a mobile station. According to one embodiment of the base station comprises a receiver capable of receiving a system access message transmitted by the mobile station, wherein the system access message initiates in the wireless network a system access procedure operable to establish a communications channel between the mobile station and the CDMA-based wireless network. The base station also comprises a handoff controller, coupled to the receiver and receiving the system access message therefrom, the handoff controller capable of transferring control over the system access procedure from the base station to a first selected base station in the CDMA-based wireless network, wherein the handoff controller sends to the first selected base station a handoff control message operable to initiate in the first selected base station a handoff procedure capable of assuming control over the system access procedure.
The present invention therefore introduces the novel concept of speculatively initiating a handoff procedure to another base station without waiting for a message from the mobile station that initiates the handoff procedure. This allows a handoff procedure to be performed more rapidly and allows the handoff procedure to be initiated while the mobile station is still in a system access state, rather than in a conversation state.
In one embodiment of the present invention, the system access message comprises signal strength indicia indicating a strength of at least one signal transmitted by at least one base station in the wireless network.
In another one embodiment of the present invention, the handoff controller sends the handoff control message to the first selected base station in response to a signal strength indicia in the system access message corresponding to a signal transmitted by the first selected base station.
In still another embodiment of the present invention, the handoff controller sends to the first selected base station a channel assignment control message, the channel assignment control message causing the first selected base station to transmit to the mobile station a first channel assignment message assigning the mobile station to operate in a selected data traffic channel.
In yet another embodiment of the present invention, the base station transmits a second channel assignment message to the mobile station assigning the mobile station to operate in the selected data traffic channel.
In yet still another embodiment of the present invention, the handoff controller sends to each of a plurality of selected base stations a handoff control message operable to initiate in the each of the plurality of selected base stations a handoff procedure capable of assuming control over the system access procedure.
In a further embodiment of the present invention, the handoff controller sends to each of a plurality of selected base stations a channel assignment control message, the channel assignment control message causing the each of the plurality of selected base stations to transmit to the mobile station a channel assignment message assigning the mobile station to operate in a selected data traffic channel.
In a still further embodiment of the present invention, the base station transmits a channel assignment message to the mobile station assigning the mobile station to operate in the selected data traffic channel.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.